This invention relates to polyurethane resins, a process for producing the same and uses thereof. More particularly, the invention relates to polyurethane resins having at least one group selected from hydrazine groups, hydrazide groups and semicarbazide groups (hereinafter, the group of these functional groups is described as HYD. groups), in the molecule, a process for producing the same, aqueous ink compositions for plastic film using the same as binders, aqueous adhesive agents for film lamination and a method of making laminates using said ink and/or adhesives.
With the increasing use of versatile flexible packagings, there has arisen a need to provide high performance printing inks, various coating agents, and adhesives that are applied for decorative, surface protecting, or other purposes. Consider, for example, the printing inks for use on plastic films, they are required to exhibit much better performance than conventional versions in various aspects such as printability, adhesion to various kinds of films, antiblocking properties, and gloss.
Particularly in the food packaging industry, laminated flexible packagings are currently used since direct contact of the contents with inks must be avoided to ensure utmost hygiene while impressing consumers with the high quality of printing. Lamination is carried out two methods described hereunder: extrusion lamination which comprises printing an ink on various plastic film bases, applying a primer coat on the printed surface as required, and then laminating a molten layer of polyolefin or the like; and adhesive lamination which comprises applying an adhesive to the printed surface, and then laminating a plastic film. Whichever method is adopted, inks to be used on the various kinds of plastic films to be laminated are required to adhere strongly not only to the base film but also to the film to be laminated.
One application of laminated flexible packagings that is growing today involves boiling or retorting in hot water so as to cook or sterilize the contents thereof; in this case, the flexible packagings must withstand boiling or retorting without suffering from delamination.
Since the various aspects of ink performance depend primarily upon the performance of binder resins, solvent-based laminating using polyurethane resins as binders have heretofore been used extensively; those inks exhibit not only strong adhesion to various kinds of films but also good adaptability for laminating.
In contrast with such solvent-based laminating inks, the demand for the use of aqueous printing inks is also increasing today in consideration of various aspects including environmental problems, labor saving, occupational safety and food hygiene. However, aqueous printing inks generally exhibit low adhesion to plastic films and the peel strength of laminates; further, their adaptability for boiling and retorting is unsatisfactory.
Under these circumstances, the assignee previously filed Japanese Patent Application No. 354568/1991, in which they proposed an aqueous laminating printing ink for use as a binder in an aqueous polyurethane resin containing a polycarbonate diol as a diol component and which was improved in adhesion to various plastic films and peel strength of laminates. Flexible packagings produced by using such aqueous laminating inks can be used to make bags for packaging dry foods but it does not have sufficient adaptability for making bags that can withstand boiling or retorting.
The assignee also filed Japanese Patent Application No. 317425/1992, in which they proposed a method for improving the adhesion to various plastic films and the peel strength of laminates by using, as an ink binder resin, an acrylic copolymer that had functional groups capable of reaction with a hydrazine group or a hydrazide group introduced into the molecule, and hydrazinc compounds as a crosslinking agent, said agent with those functional groups and carbonyl groups that developed on the film surface by subsequent surface treatment. However, compared to inks using polyurethane-base binder resins that can be used and, hence, the printing inks used in this method are poor in pigment dispersability and printability; furthermore, during storage, the binder resins will crosslink with the crosslinking agents to lower their fluidity and capability for redissolution. As another problem, there is no guarantee for the occurrence of positive crosslinking said agents with the binder resins and the film surface, and this has often caused nonuniformity in adhesion and the peel strength of laminates.
Thus, no aqueous printing inks have yet been commercialized that are satisfactory not only in adhesion to various kind of plastic films but also in the peel strength of laminates, as well as in adapatability for boiling and retorting.
The advent of aqueous primer coat or adhesives is desired to insure that laminated flexible packagings are totally consisted of water, thereby achieving occupational safety and food hygiene; however, no acceptable products have yet been commercialized.